1. Field of the Invention
The invention relates to a scanning line number converting circuit for converting video signals of various formats of the different numbers of scanning lines into video signals of the predetermined number of scanning lines.
2. Description of the Related Art
As a standard television broadcasting signal, an NTSC (National Television System Committee) system and a PAL (Phase Alternation by Line) system have been known. Although the number of scanning lines of one frame is equal to 525 in the NTSC system, the number of scanning lines of one frame is equal to 625 in the PAL system. The numbers of scanning lines, therefore, in the NTSC system and the PAL system differ.
The development of television broadcastings of not only the standard system such as NTSC system or PAL system but also the HDTV (High Definition Television) system has been being developed in recent years. The number of scanning lines of one frame in the HDTV system is equal to 1125.
In computer images, further, a video signal of a format different from that of the television broadcasting is used, the number of pixels of VGA (Video Graphics Array) is equal to (640.times.480) dots, and the number of pixels of SVGA (Super VGA) is equal to (800.times.600)dots.
As mentioned above, in recent years, not only the video signal of the standard system such as NTSC system or PAL system but also video signals of various formats of the different numbers of scanning lines such as video signal of the HDTV system, video signal for computers, and the like are used. A display which can cope with the video signals of those various formats is demanded.
Hitherto, as a display, a CRT (Cathode Ray Tube) display has widely been used. In case of the CRT display, the number of scanning lines changes by changing a deflecting speed of an electron beam. It is, therefore, possible to relatively easily realize a display which can cope with the video signals of various formats. However, in the CRT display, an electric power consumption is large and it is difficult to miniaturize.
On the other hand, recently, in place of the CRT display, the development of an LCD (Liquid Crystal Display) display, a plasma display, and the like has been being progressed. In the LCD display and plasma display, since the size is small and an electric power consumption is small, it is presumed that they will be further spread in future.
In the LCD display and plasma display, however, the positions and the number of pixels are fixed. Therefore, to allow the LCD display or plasma display to cope with video signals of various formats, it is necessary to convert the number of scanning lines.
As a method of converting the number of scanning lines, there have been proposed a nearest neighborhood interpolating method of extracting data of a line existing at the position nearest to the position of a line after completion of the conversion of the number of scanning lines from inputted data of one scanning line, a bilinear interpolating method of extracting data of two lines existing at the positions nearest to the position of the line after completion of the conversion of the number of scanning lines from inputted data of one scanning line and linearly interpolating from the data of two lines, a filter switching interpolating method of converting the number of scanning lines by using an FIR filter in accordance with a conversion ratio, and the like.
Although the nearest neighborhood interpolating method can be realized by an extremely simple logic arithmetic operation on a hardware construction, there are problems such that a picture quality after the conversion fairly deteriorates, thin lines are extinguished and a small figure is distorted at the time of reduction, and a notched portion appears in a peripheral portion at the time of enlargement.
According to the bilinear interpolating method, although the deterioration of the picture quality is less than that of the nearest neighborhood interpolating method, when an image is reduced into (2:1) or less, a phenomenon called a line dropout occurs and the picture quality remarkably deteriorates. According to this method, since a gentle low pass filter is performed, particularly, a picture quality of a vertical edge portion (lateral fringe) becomes a picture quality of a slightly blur image.
On the other hand, in the filter switching interpolating method, the conversion of the number of scanning lines is performed by using an FIR filter in accordance with a conversion ratio. According to the filter switching interpolating method, although a construction becomes complicated, the conversion of the number of scanning lines can be performed at a high picture quality as compared with the nearest neighborhood interpolating method and the bilinear interpolating method.
The filter switching interpolating method will now be described hereinbelow. The conversion of the number of scanning lines of a non-interlace image will be first described. In the non-interlace image, a process of a frame period is performed and even after the conversion of the number of scanning lines, so long as the non-interlace, there is no need to separate the processes for the first field and the second field. Therefore, the processes are relatively more simple than those of an interlace image. To show an outline of an idea, explanation will be first made with respect to a non-interlace signal as an example.
For example, a principle of the conversion of the number of lines for the (2:3) enlargement (hereinafter, also referred to as a (2:3) enlargement line number conversion) such as to form three output lines for two input lines will now be described.
FIG. 1 shows a diagram for explaining the principle of the line number conversion for the (2:3) enlargement. In FIG. 1, values of each input line are set to Ri-1, Ri, Ri+1, Ri+2, Ri+3, . . . and values of each output line are set to Qi, Qi+1, Qi+2, Qi+3, . . . , respectively. In the diagram, P1, P2, P3, P1, . . . indicate deviations (line phase information) of the phases of the input lines and output lines.
In the (2:3) enlargement line number conversion, as shown in FIG. 1, three output lines are formed for two input lines and between the input line and the output line, there is a relation such that the values of the output line are calculated from the input line near them. Various interpolating methods exist in dependence on which range is used as a neighborhood range to form the output lines, which coefficient values are used as values of coefficients when the output lines are calculated by the interpolation from the input lines, or the like. However, an example of a cubic interpolation for interpolating from ranges of four points (corresponding to four lines) as neighborhood ranges will now be described hereinbelow.
A cubic interpolation function Cub(x) which is used in the cubic interpolation is shown in FIG. 2 and its functional equations are shown in equations (1). It is assumed that an axis of abscissa of the cubic interpolation function shown in the equations (1) is normalized by a sampling interval when an original image is sampled to a digital signal. EQU Cub(x)=.vertline.x.vertline..sup.3 -2.vertline.x.vertline..sup.2 +1 (when .vertline.x.vertline..ltoreq.1) EQU Cub(x)=-.vertline.x.vertline..sup.3 +5.vertline.x.vertline..sup.2 -8.vertline.x.vertline.+4 (when 1&lt;.vertline.x.vertline..ltoreq.2) EQU Cub(x)=0 (when 2&lt;.vertline.x.vertline.) (1)
In case of the enlargement line number conversion, an interpolation value of each output line is expressed by a convolution arithmetic operation of the values of the four input lines and the cubic function and the interpolation values of the output lines can be expressed as shown by the following equations (2). EQU Qi=Cub(x11)*Ri-1+Cub(x12)*Ri+Cub(x13)*Ri+1+Cub(x14)*Ri+2 EQU Qi+1=Cub(x21)*Ri-1+Cub(x22)*Ri+Cub(x23)*Ri+1+Cub(x24)*Ri+2 EQU Qi+2=Cub(x31)*Ri+Cub(x32)*Ri+1+Cub(x33)*Ri+2+Cub(x34)*Ri+3 (2)
Each coefficient Cub(x) of the equations (2) is a value that is calculated from the cubic interpolation function and is calculated from the phase showing a degree of the deviation of the output line to be obtained from the input line. For example, in case of the (2:3) enlargement line number conversion shown in FIG. 1, since the phase of the output line Qi coincides with the phase of the input line (for example, input line Ri) near it, the phase information P1 is equal to zero. Similarly, since the phase of the output line Qi+1 is deviated from the phase of the input line (input line Ri) near it by 2/3, the phase information P2 is equal to 2/3. Since the phase of the output line Qi+2 is deviated from the phase of the input line (for example, input line Ri+1) near it by 1/3, the phase information P3 is equal to 1/3. Therefore, the equations (2) can be rewritten as shown in the following equations (3). EQU Qi=Cub(-1)*Ri-1+Cub(0)*Ri+Cub(1)*Ri+1+Cub(2)*Ri+2 EQU Qi+1=Cub(-5/3)*Ri-1+Cub(-2/3)*Ri+Cub(1/3)*Ri+1+Cub(4/3)*Ri+2 EQU Qi+2=Cub(-4/3)*Ri+Cub(-1/3)*Ri+1+Cub(2/3)*Ri+2+Cub(5/3)*Ri+3 (3)
Since the coefficient Cub(x) and the values Ri-1, Ri, Ri+1, and Ri+2 of the input lines are well-known values, the interpolation data of each output line can be calculated from the equations (3). For example, when considering only the output line Qi, since Cub(-1)=0, Cub(0)=1, Cub(1)=0, and Cub(2)=0 from the equations (1), EQU Qi=0*Ri-1+1*Ri+0*Ri+1+0*Ri+2=Ri (4)
Thus, it becomes the value of the input line itself.
Although the case of the (2:3) enlargement line number conversion has been described as an example above, the same shall also similarly apply to an arbitrary enlargement ratio. If the phase of the output line is merely known, each coefficient of the cubic function is obtained from the equations (1) by the phase and it is sufficient to perform a convolution arithmetic operation with the four input lines near the interpolation line.
A principle of, for instance, the conversion of the number of lines for a (3:2) reduction (hereinafter, also referred to as a (3:2) reduction line number conversion) such as to form two output lines for three input lines will now be explained.
FIG. 3 shows a diagram for explaining the principle of the (3:2) reduction line number conversion. Even in FIG. 3 as well, in a manner similar to FIG. 1, values of each input line are set to Ri-1, Ri, Ri+1, Ri+2, Ri+3, . . . and values of each output line are set to Qi, Qi+1, Qi+2, . . . , respectively. In the diagram, P1, P2, P1, . . . indicate deviations (line phase information) of the phases of the input lines and output lines.
In the (3:2) reduction line number conversion, in a manner similar to the enlargement line number conversion, between the input line and the output line, there is a relation such that the values of the output line are calculated from the input line near them. Even in the (3:2) reduction line number conversion, a cubic interpolation for calculating the output line (interpolation line) by the interpolation from the four input lines near it in a manner similar to the above will now be described.
That is, in case of the reduction line number conversion of FIG. 3, interpolation equations of the interpolation values (for example, Qi, Qi+1) of each output line are as shown in the following equations (5). EQU Qi=Cub(x11)*Ri-1+Cub(x12)*Ri+Cub(x13)*Ri+1+Cub(x14)*Ri+2 EQU Qi+1=Cub(x21)*Ri+Cub(x22)*Ri+1+Cub(x23)*Ri+2+Cub(x24)*Ri+3 (5)
Even in the reduction line number conversion, each coefficient Cub(x) of the equations (5) is a value that is calculated from the equations (1) as cubic functional equations and is calculated from the phase showing a degree of the deviation of the output line to be obtained from the input line. In case of the (3:2) reduction line number conversion shown in the diagram, since the phase of the output line Qi coincides with the phase of the input line (input line Ri) near it, the phase information P1 is equal to zero. Similarly, since the phase of the output line Qi+1 is deviated from the phase of the input line (input line Ri+1 of Ri) near it by 1/2, the phase information P2 is equal to 1/2. Therefore, the equations (5) can be rewritten as shown in the following equations (6). EQU Qi=Cub(-1)*Ri-1+Cub(0)*Ri+Cub(1)*Ri+1+Cub(2)*Ri+2 EQU Qi+1=Cub(-3/2)*Ri+Cub(-1/2)*Ri+1+Cub(1/2)*Ri+2+Cub(3/2)*Ri+3 (6)
Since the coefficient Cub(x) and the values Ri-1, Ri, Ri+1, and Ri+2 of the input lines are well-known values, the interpolation data of each output line can be calculated from the equations (6). For example, when considering only the output line Qi, since Cub(-1)=0, Cub(0)=1, Cub(1)=0, and Cub(2)=0 from the equations (1), EQU Qi=0*Ri-1+1*Ri+0*Ri+1+0*Ri+2=Ri (7)
Thus, it becomes the value of the input line itself.
Although the case of the (3:2) reduction line number conversion has been described as an example above, the same shall also similarly apply to an arbitrary reduction ratio. If the phase of the output line is merely known, each coefficient of the cubic function is obtained from the equations (1) by the phase and it is sufficient to perform a convolution arithmetic operation with the four input lines near the interpolation line.
Hitherto, the line number conversion as mentioned above has been realized by, for example, a hard wired construction as shown in FIG. 4. In the line number conversion, the process to distinguish a luminance signal and a chroma signal in accordance with a format of the chroma as in the pixel number conversion is unnecessary and it is sufficient to use the same circuit for the luminance signal and for the chroma signal.
In the construction shown in FIG. 4, each of line memories (or registers) 101 to 104 which are serially connected delays supplied data by the time corresponding to one scanning line. Therefore, the line memories of four stages are constructed by them. In the line memories 101 to 104, when an input shift control signal IE is at the "H" level, the input data of one line supplied from an input terminal 100 is delayed and image data which was shifted by the time of one scanning line is outputted. In the registers 101 to 104, on the other hand, when the input shift control signal IE is at the "L" level, the input data is not shifted but the previous line value is held. The image data obtained by line-shifting by the registers 101 to 104 is sent to corresponding multipliers 111 to 114, respectively.
A cubic coefficient generator 105 generates cubic coefficients C1 to C4 every line and supplies the cubic coefficients C1 to C4 as multiplication coefficients to the corresponding multipliers 111 to 114, respectively. In the multipliers 111 to 114, therefore, the cubic coefficients C1 to C4 generated by the cubic coefficient generator 105 and the input line data shifted by the shift registers 101 to 104 are multiplied, respectively. However, the values of the cubic coefficients generated by the cubic coefficient generator 105 are switched every line and are set to the same value in one line. Multiplication results of the multipliers 111 to 114 are added by an adder 107 and an addition result is inputted to an FIFO (first-in first-out) field memory 108.
A field memory 110 is provided to desultorily output the line data necessary in case of the enlargement line number converting process. In case of the enlargement line number conversion, a mode to output the line data or a mode to hold the value of the previous line is switched on the basis of an input skip line control signal SCI which is supplied from a controller 106 and the line data or the value of the previous line is outputted to the line memory 101. In case of the reduction line number converting process, the field memory 110 is used as a mere FIFO memory and is a mere delay element.
A field memory 108 is provided to desultorily output the line data in case of the reduction line number converting process. In case of the reduction line number conversion, the line data is desultorily skipped on the basis of an output skip line control signal SCO which is supplied from the controller 106 and is outputted to an output terminal 109. The field memory 108 is used as a mere FIFO memory in case of the enlargement line number converting process and is a mere delay element.
The controller 106 generates the output skip line control signal SCO of the field memory 108 as an output port memory and the input shift control signal IE of the line memories 101 to 104 on the basis of the conversion ratio when the enlargement or reduction line number conversion is performed and, further, performs a timing control for the cubic coefficient generator 105.
FIG. 5 shows relations between a line layout at the time of the (2:3) enlargement line number converting process and the cubic coefficients C1, C2, C3, and C4 in the hardware construction of FIG. 4. FIG. 6 shows the cubic coefficients at each phase. In case of performing the (2:3) enlargement line number converting process, as shown in FIG. 5, the operations such that the input line data as much as three lines is shifted by the input shift control signal IE and the line data of one line before is not shifted are repeated. Input data D1, D2, D3, and D4 to the multipliers 111 to 114 in FIG. 4 become multiplier inputs D1, D2, D3, and D4 in FIG. 5. As shown in the following equation (8), a desired result is derived by performing a convolution arithmetic operation of those multiplier inputs and the cubic coefficients C1, C2, C3, and C4. EQU Q=C1*D1+C2*D2+C3*D3+C4*D4 (8)
Although the example of the (2:3) enlargement line number conversion has been shown here for simplicity of explanation, in case of arbitrary enlargement ratios, since their principles are substantially the same as that mentioned above except that the timing control merely differs, their descriptions are omitted here.
FIG. 7 shows relations between a line layout at the time of the (3:2) reduction line number converting process and the cubic coefficients C1, C2, C3, and C4 in the hardware construction of FIG. 4. FIG. 8 shows the cubic coefficients at each phase. In the diagram, "Skip" denotes output lines to be skipped. In case of the reduction line number converting process, different from the case of the enlargement line number converting process, the input shift control signal IE is always set to the "L" level and the input line data is inputted as it is to the registers 101 to 104. Therefore, the input data D1 to D4 of the multipliers 111 to 114 become multiplier inputs D1 to D4 in the diagram. A desired result is derived by performing a convolution arithmetic operation of those multiplier inputs and the cubic coefficients C1, C2, C3, and C4 in accordance with the equation (8). In case of the (3:2) reduction line number conversion, since one input line is unnecessary for three lines to be outputted, the unnecessary line is skipped by controlling the writing operation to the field memory 108. A control signal for this purpose becomes the skip control signal SCO of the output line. That is, the output skip line control signal SCO is a signal to control the field FIFO memory 108 in a manner such that the lines are skipped when the signal SCO is at the "H" level and the lines are not skipped when it is at the "L" level.
Although the example of the (3:2) reduction line number converting process has been shown here for simplicity of explanation, in case of arbitrary reduction ratios, since their principles are substantially the same as that mentioned above except that the timing control merely differs, their descriptions are omitted here.
The above explanation relates to the example in case of what is called a non-interlace signal in which the input signal is subjected to the progressive scan. In what is called an interlace signal of the interlace scan, since the positions of the scanning lines are different in the first field and the second field for the picture plane, a setting method of the coefficients for interpolation differs for each of the second and first fields. Therefore, the actual control system has a more complicated construction and if the ratio differs, the coefficients also naturally differ, so that the input lines to be skipped and the output lines to be skipped also change in accordance with them. It is, therefore, necessary to independently calculate the interpolation coefficients and the skip line information in each field.
The conversion of the number of scanning lines (hereinafter, also referred to as a scanning line number conversion) of the interlace signal will now be described.
In the (2:3) enlargement scanning line number conversion, interpolation equations of the first field are substantially the same as the equations (3) in which the input is the non-interlace signal. In the interlace signal, like a first field input line signal 121 and a second field input line signal 123 and like a first field output line signal 122 and a second field output line signal 124 in FIG. 9, a relation in which the phases of the first and second fields are deviated by 1/2 has to be obtained, namely, the line of the second field has to be arranged at the center between the lines of the first field. Therefore, in the second field of the (2:3) enlargement scanning line number conversion, the phase deviations of the interpolation lines become P4, P5, and P6 in FIG. 9 and the phase information is equal to 5/7, 1/2, and 1/7, respectively. Thus, the interpolation values of the lines can be written as shown by the following equations (9) in a manner similar to the equations (3). EQU Qj=Cub(-12/7)*Rj-2+Cub(-5/7)*Rj-1+Cub(2/7)*Rj+Cub(9/7)*Rj+1 EQU Qj+1=Cub(-3/2)*Rj-1+Cub(-1/2)*Rj+Cub(1/2)*Rj+1+Cub(3/2)*Rj+2 EQU Qj+2=Cub(-8/7)*Rj+Cub(-1/7)*Rj+1+Cub(6/7)*Rj+2+Cub(13/7)*Rj+3 (9)
A suffix "j" denotes the second field and is distinguished from the suffix "i" of the first field. As will be also understood from the above equations, since there is no relation between the cubic coefficients and the lines necessary for interpolation for the first field and those for the second field, it is necessary to independently calculate them for an image signal of the interlace format.
Similarly, in the (3:2) reduction scanning line number conversion, interpolation equations of the first field are substantially the same as the equations (6) in which the input is the non-interlace signal. In this case as well, as shown in FIG. 10, like a first field input line signal 125 and a second field input line signal 127 and like a first field output line signal 126 and a second field output line signal 128, a relation in which the phases of the first and second fields are certainly deviated by 1/2 has to be obtained, namely, the line of the second field has to be arranged at the center between the lines of the first field. Therefore, in the second field of the (3:2) reduction scanning line number conversion, the phase deviations of the interpolation lines become P3 and P4 in FIG. 10 and the phase information is equal to 1/4 and 3/4, respectively. Thus, the interpolation values of the lines can be written as shown by the following equations (10) in a manner similar to the equations (6). EQU Qj=Cub(-5/4)*Rj-1+Cub(-1/4)*Rj+Cub(3/4)*Rj+1+Cub(7/4)*Rj+2 EQU Qj+1=Cub(-7/4)*Rj+Cub(-3/4)*Rj+1+Cub(1/4)*Rj+2+Cub(5/4)*Rj+3 (10)
In this case as well, since the cubic coefficient set as for the second field is quite different from that in the first field, it is necessary to independently calculate them.
In the scanning line number converting process, it has been realized hitherto by what is called an ASIC (Application Specific Integrated Circuit) as mentioned above or the like.
In the scanning line number conversion as mentioned above, the interpolation coefficients and skip lines are totally different in dependence on whether the input signal is the non-interlace signal or the interlace signal and, further, in case of the interlace signal, they are different every field. To realize the interpolation arithmetic operation which is calculated from the coefficients of multi-taps like a cubic interpolation by ASIC, a conversion ratio of a small degree of freedom or a certain fixed conversion ratio has to be used or it cannot help limiting to a system such that up to a few kinds of conversion ratios are switched and used in consideration of a viewpoint of a circuit scale.
To cope with various ratios, further, to cope with various formats, or from a viewpoint of flexibility or the like such as change in bit precision after completion of the design, change in ratio converting algorithm, or the like, it is difficult to realize it by only hardware such as ASIC.
It is actually impossible to change the horizontal and vertical conversion in a real-time manner by the ASIC by using a filter switching interpolating method that is complicated on the circuit construction.
As mentioned above, in the conventional scanning line number converting circuit, the field memories and a memory controller are necessary. There are problems such that the field memories and the memory controller are relatively expensive and if the field memories and memory controller are provided, the circuit scale increases.